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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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CLOCK(3P) POSIX Programmer's Manual CLOCK(3P)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
clock — report CPU time used
#include <time.h>
clock_t clock(void);
The functionality described on this reference page is aligned with
the ISO C standard. Any conflict between the requirements
described here and the ISO C standard is unintentional. This
volume of POSIX.1‐2017 defers to the ISO C standard.
The clock() function shall return the implementation's best
approximation to the processor time used by the process since the
beginning of an implementation-defined era related only to the
process invocation.
To determine the time in seconds, the value returned by clock()
should be divided by the value of the macro CLOCKS_PER_SEC.
CLOCKS_PER_SEC is defined to be one million in <time.h>. If the
processor time used is not available or its value cannot be
represented, the function shall return the value (clock_t)-1.
No errors are defined.
The following sections are informative.
None.
In programming environments where clock_t is a 32-bit integer type
and CLOCKS_PER_SEC is one million, clock() will start failing in
less than 36 minutes of processor time for signed clock_t, or 72
minutes for unsigned clock_t. Applications intended to be
portable to such environments should use times() instead (or
clock_gettime() with CLOCK_PROCESS_CPUTIME_ID, if supported).
In order to measure the time spent in a program, clock() should be
called at the start of the program and its return value subtracted
from the value returned by subsequent calls. The value returned by
clock() is defined for compatibility across systems that have
clocks with different resolutions. The resolution on any
particular system need not be to microsecond accuracy.
None.
None.
asctime(3p), clock_getres(3p), ctime(3p), difftime(3p),
gmtime(3p), localtime(3p), mktime(3p), strftime(3p), strptime(3p),
time(3p), times(3p), utime(3p)
The Base Definitions volume of POSIX.1‐2017, time.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 by the Institute of Electrical and Electronics Engineers,
Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard,
the original IEEE and The Open Group Standard is the referee
document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page
are most likely to have been introduced during the conversion of
the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 CLOCK(3P)
Pages that refer to this page: time.h(0p), asctime(3p), ctime(3p), difftime(3p), gmtime(3p), localtime(3p), mktime(3p), strftime(3p), time(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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